Memory cell with self-aligned floating gate and separate select gate, and fabrication process

ABSTRACT

Memory cell having a floating gate with lateral edges which are aligned directly above edges of the active area in the substrate, a control gate positioned directly above the floating gate, and a select gate spaced laterally from the control gate. The floating gate has a bottom wall and side walls which face corresponding walls of the control gate in capacitive coupling relationship, with the height of the side walls being on the order of 80 to 160 percent of the width of the bottom wall. In some embodiments, the floating gate is wider than the overlying control gate and has projecting portions which overlie the source and drain regions of the stack transistor. The memory cell is fabricated by forming a poly-1 layer and an overlying dielectric film on a substrate in areas in which the stack transistors are to be formed, forming a poly-2 layer over the dielectric film and over areas of the substrate in which the select transistors are to be formed, patterning the poly-2 layer to form control gates for the stack transistors and select gates for the select transistors, removing the poly-1 layer and the dielectric film to form floating gates in areas which are not covered by the control gates, and forming source and drain regions in the substrate. The floating gates are aligned with active areas in the substrate by forming isolation oxide regions which extend above the substrate at the edges of the active areas, and forming the floating gates on the sides of the isolation oxide regions in alignment with the edges of the active areas.

BACKGROUND FIELD OF INVENTION

This invention pertains generally to semiconductor devices and, moreparticularly, to a nonvolatile memory device and fabrication process.

RELATED ART

Electrically programmable read only memory (EPROM) has been widely usedas nonvolatile memory which can keep data unchanged even though thepower is turned off. However, EPROM devices have a major disadvantage inthat they have to be exposed to Ultra-Violet (UV) light for about 20minutes for data erasure. This is very inconvenient because an EPROMdevice has to be unplugged from its socket and moved to the UV lightsource when the data needs to be changed.

Electrically erasable programmable read only memory (EEPROM) overcomesthis problem and permits data to be erased electrically in a muchshorter period of time, typically less than 2 seconds. However, it stillhas a disadvantage in that the data must be erased on a byte-by-bytebasis.

Flash EEPROM is similar to EEPROM in that data is erased electricallyand relatively quickly. However, with flash EEPROM, the data is erasedin blocks which typically range in size from 128 to 64K bytes per block,rather than on a byte-by-byte basis.

In general, there are two basic types of nonvolatile memory cellstructures: stack-gate and split-gate. The stack-gate memory cellusually has a floating gate and a control gate, with the control gatebeing positioned directly above the floating gate. In a split-gate cellthe control gate is still positioned above the floating gate, but it isoffset laterally from it. The fabrication process for a stack-gate cellis generally simpler than that for a split-gate cell. However, astack-gate cell has an over-erase problem which a split-gate cell doesnot have. This problem is commonly addressed by maintaining thethreshold voltage of the cell in a range of about 0.5-2.0 volts after anerase cycle, which adds complexity to the circuit design.

A split-gate memory cell has an additional gate known as a select gatewhich avoids the over-erase problem and makes circuit design relativelysimple. Such cells are typically fabricated in double-poly ortriple-poly processes which are relatively complex, and they are moresusceptible to various disturbances during programming and readoperations.

EEPROM devices have typically included a stack-gate transistor and aseparate select gate transistor. With no over-erase problem, circuitdesign has been relatively simple, but these devices have a relativelyhigh die cost due to larger cell size as compared to split-gate andstack-gate memory cells.

A memory cell is erased by forcing electrons to migrate away from thefloating gate so that it becomes charged with positive ions. This iscommonly accomplished by Fowler-Nordheim tunneling in which a tunneloxide having a thickness on the order of 70-120 Å is formed between themonocrystalline silicon substrate and the floating gate. A relativestrong electric field (greater than 10 mV/cm) is then applied to thetunnel oxide, and the electrons tunnel from the floating gate toward theunderlying source, drain or channel region. This technique is widelyused both in stack-gate cells and in split-gate cells, and is describedin greater detail in U.S. Pat. Nos. 5,792,670, 5,402,371, 5,284,784 and5,445,792.

Another way of forming an erase path is to grow a dielectric filmbetween two polysilicon (poly-Si) layers as a tunneling dielectric. U.S.Pat. No. 5,029,130 discloses the formation of a sharp edge on thefloating gate to enhance the local electric field around it, with theerase path being formed between the sharp edge and the control gate. Byadding a third polycrystalline silicon layer as an erase layer whichcrosses over, or overlies, the floating gate and the control gate, anerase path can be formed between the side wall of floating gate and theerase layer. This technique is disclosed in U.S. Pat. Nos. 5,847,996 and5,643,812.

Fowler-Nordheim tunneling can also be used to program a memory cell byforcing electrons to tunnel into the floating gate so that it becomescharged negatively. U.S. Pat. Nos. 5,792,670 and 5,402,371 show examplesin which electrons are forced to tunnel into the floating gate from thechannel region beneath it.

Another way of programming a memory cell is by the use of channel hotcarrier injection. During a programming operation, the electrons flowingfrom the source to the drain are accelerated by a high electric fieldacross the channel region, and some of them become heated near the drainjunction. Some of the hot electrons exceed the oxide barrier height andare injected into floating gate. This technique is found in U.S. Pat.No. 4,698,787.

FIG. 1 illustrates a prior art NOR-type flash EEPROM cell array in whichthe floating gates 16 have end caps 16 a, 16 b which extend over theadjacent isolation oxide regions 19. The floating gate is typically madeof polysilicon or amorphous silicon with a thickness on the order of1500-2500 Å. Control gates 21 cross over the floating gates, and aretypically made of heavily doped polysilicon or polycide. Select gates 22are separated from and parallel to the control gates. Bit lines 23,which are typically formed by a metallization layer, interconnect all ofthe drains of the memory cells in the respective columns, with adjacentones of the bit lines being isolated from each other. All of the sourcesof the memory cells in a given row are connected together by a commonsource line 24 which is typically formed by an N+ or a P+ diffusionlayer in the single crystalline silicon substrate.

The floating gate end caps 16 a, 16 b are required because of acorner-rounding effect or a shift of the floating gate which occursduring the photolithographic step by which the floating gate is formed.The corner-rounding effect may make the edges 16 c, 16 d of the floatinggate shorter, and the shift of the floating gate may make one or both ofthe edges 16 c, 16 d move beyond the edges 28 a, 28 b of active area 28.Both of these effects can cause malfunction of the memory cell because aleakage path may occur when the floating gate does not completely coverthe active area or its channel length becomes too short.

FIGS. 2A and 2B illustrate the memory cell array of FIG. 1 with shallowtrench and LOCOS (local oxidation of silicon) isolation, respectively.As seen in these figures, an inter-poly dielectric film 31 is formedbetween the conduction layers which form the floating gates 16 and thecontrol gates 21. Those layers are commonly referred to as the poly-1and poly-2 layers, respectively, and the dielectric film is typicallyformed of either pure oxide or a combination of oxide and nitride films.

The end caps 16 a, 16 b which extend over the adjacent isolation oxideregions 19 help in the formation of large capacitance areas between thecontrol gates 21 and the floating gates 16. Consequently, the couplingratio from the control gate to the floating gate becomes large, and thismakes it possible to couple more voltage from the control gate to thefloating gate during programming and erase operations. In order toinsure that the floating gate will completely cover the active area andthat the channel length will not become too short due to variationsduring the fabrication process, it is necessary to add tolerance to thememory cell layout by making the floating gate caps wider. In addition,the distance 32 between the end caps has to be kept wide enough to avoidshorts from developing between the floating gates. As a result, the sizeof the memory cell increases, and the cost gets higher.

OBJECTS AND SUMMARY OF THE INVENTION

It is in general an object of the invention to provide a new andimproved memory cell and process for fabricating the same.

Another object of the invention is to provide a memory cell and processof the above character which overcome the limitations and disadvantagesof the prior art.

These and other objects are achieved in accordance with the invention byproviding a memory cell having a floating gate with lateral edges whichare aligned directly above edges of the active area in the substrate, acontrol gate positioned directly above the floating gate, and a selectgate spaced laterally from the control gate. The floating gate has abottom wall and side walls which face corresponding walls of the controlgate in capacitive coupling relationship, with the height of the sidewalls being on the order of 80 to 160 percent of the width of the bottomwall. In some embodiments, the floating gate is wider than the overlyingcontrol gate and has projecting portions which overlie the source anddrain regions of the stack transistor.

The memory cell is fabricated by forming a poly-1 layer and an overlyingdielectric film on a substrate in areas in which the stack transistorsare to be formed, forming a poly-2 layer over the dielectric film andover areas of the substrate in which the select transistors are to beformed, patterning the poly-2 layer to form control gates for the stacktransistors and select gates for the select transistors, removing thepoly-1 layer and the dielectric film to form floating gates in areaswhich are not covered by the control gates, and forming source and drainregions in the substrate. The floating gates are aligned with activeareas in the substrate by forming isolation oxide regions which extendabove the substrate at the edges of the active areas, and forming thefloating gates on the sides of the isolation oxide regions in alignmentwith the edges of the active areas.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top plan view of a prior art NOR-type flash EEPROM memorycell array.

FIGS. 2A and 2B are enlarged, fragmentary cross-sectional views takenalong line 2—2 of FIG. 1.

FIG. 3 is a top plan view of one embodiment of a NOR-type flash EEPROMmemory cell array fabricated in accordance with the invention.

FIGS. 4A-4F are enlarged, schematic cross-sectional views taken alongline 4—4 of FIG. 3, illustrating the steps in one embodiment of aprocess for fabricating the memory cell array of FIG. 3.

FIGS. 5A-5C are schematic cross-sectional views similar to FIGS. 4A -4F,illustrating the steps in another embodiment of a process forfabricating the memory cell array of FIG. 3.

FIG. 6 is a top plan view of another embodiment of a NOR-type flashEEPROM memory cell array fabricated in accordance with the invention.

FIGS. 7A, and 7B are enlarged, schematic cross-sectional views takenalong line 7—7 of FIG. 6, illustrating the steps in one embodiment of aprocess for fabricating the memory cell array of FIG. 6.

FIGS. 8A and 8B are enlarged cross-sectional views taken along lines 8—8of FIGS. 3 and 6.

FIG. 9 is a circuit diagram of the memory cell arrays of FIGS. 3 and 6.

DETAILED DESCRIPTION

As illustrated in FIG. 3, a NOR-type flash EEPROM memory cell arrayfabricated in accordance with the invention has floating gates 41 withtwo edges 41 a, 41 b which are self-aligned with the edges 42 a, 42 b ofthe active areas 42. The end caps of the prior art devices areeliminated, and as discussed more fully hereinafter, the control gates43 and the select gates 44 are defined simultaneously in a singlephotolithographic masking step. The other two edges 41 c, 41 d of thefloating gates are defined after the side edges 43 a, 43 b of thecontrol gates are formed, and the floating gates are wider than thecontrol gates. With the self-aligned floating gates, cell size and diecost are both greatly reduced.

As illustrated in FIG. 4A, the memory cell is fabricated on a siliconsubstrate 46 which can be an N-well, P-well or P-substrate material. Anoxide layer 47 having a thickness on the order of 70-120 Å is thermallygrown on the substrate to form the gate oxides of the floating gatetransistors. A conduction layer 48 of polysilicon or amorphous silicon(poly-1) having a thickness on the order of 100-1000 Å is deposited onthe thermal oxide. The poly-1 layer is doped with phosphorus, arsenic orboron to a level on the order of 10¹⁷ to 10²⁰ per cm³ either in-situduring deposition of the silicon or by ion implantation. A dielectricfilm 49 is then formed on the poly-1 layer. This film can be either apure oxide or a combination of oxide and nitride, and in one presentlypreferred embodiment, it consists of a lower oxide layer having athickness on the order of 30-100 Å, a central nitride layer having athickness on the order of 60-300 Å, and an upper oxide layer having athickness on the order of 30-100 Å.

A photolithographic mask (not shown) is then formed over the areas inwhich stack transistors are to be formed, and the poly-1 layer and thedielectric film are then etched away in the areas in which selecttransistors are to be formed, as illustrated in FIG. 4B. Another thermaloxidation is then performed to form the gate oxide 47 a for the selecttransistors. That oxide preferably has a thickness on the order of150-350 Å.

Referring now to FIG. 4C, a second polysilicon layer 51 (poly-2) isdeposited across the wafer to form the conduction layer 51 a, 51 b forthe control gates and the select gates. The poly-2 layer has a thicknesson the order of 1500-3000 Å, and is doped with phosphorus, arsenic orboron to a level on the order of 10²⁰ to 10²¹ per cm³. If desired, apolycide film can be formed on the poly-2 layer to reduce its sheetresistance. A dielectric film 52 of oxide or nitride is then depositedon the poly-2 layer.

A photolithographic mask (not shown) is positioned over dielectric film52 to define the control gates and the select gates, and an anisotropicetch is performed to remove film 52 and the poly-2 layer in the unmaskedareas, leaving the structure shown in FIG. 4D in which control gates 43and select gates 44 are formed. The poly-1 layer which forms thefloating gates is protected by dielectric layer 49 and is not etched atthis time.

An oxide film is then deposited across the wafer, and then removed fromthe flat areas in an anisotropic dry etch to form oxide spacers 53 whichsurround the control gates and select gates, as shown in FIG. 4E.

Referring now to FIG. 4F, using the control gates and the oxide spacersas a mask, the floating gates 41 are formed by etching away thedielectric film 49 and the poly-1 material which are not covered by themask. The oxide spacers are then widened by depositing an oxide film andetching it away anisotropically. Source and drain regions 56-58 are thenformed by ion implantation, with the junction depth of the sourceregions 58 of the stack transistors being made greater to withstand therelatively high voltages applied to the source nodes during eraseoperations.

With the floating gates being wider than the overlying control gates, anerase path or window is formed between one protruding portion of each ofthe floating gates and the underlying source region 58 a. The otherprotruding portion is positioned above the drain region 57 a of thestack transistor.

FIGS. 5A-5C illustrate an alternate embodiment for processing the cellarray after it has reached the point shown in FIG. 4D. In thisembodiment, a poly-oxide layer 59 is formed by thermal oxidation on theside walls of the control gates and the select gates to a thicknesswhich is preferably on the order of about 100-400 Å. Using the controlgates and the poly-oxide layers as a mask, the floating gates 41 areformed by etching away the dielectric film 49 and the poly-1 materialoutside the masked area, as shown in FIG. 5B. Thereafter, oxide spacers61 are formed around the select gates and the control gates. In thisembodiment, the spacers surround the floating gates as well as thecontrol gates. Source and drain regions 56-58 are formed byion-implantation, and the source junctions 58 is made deeper towithstand the high voltages that are applied to the source nodes duringerase operations.

The embodiment of the NOR-type flash EEPROM memory cell arrayillustrated in FIG. 6 is similar to the embodiment of FIG. 3 in that theedges 41 a, 41 b of the floating gates are self-aligned with the edges42 a, 42 b of the active areas 42, and the control gates 43 and selectgates 44 are defined simultaneously in a single photolithographicmasking step. However, it differs in that the other two edges 41 c, 41 dof the floating gates are aligned with the side edges 43 a, 43 b of thecontrol gates, rather than having the floating gates be wider than thecontrol gates.

This embodiment is fabricated in accordance with the steps illustratedin FIGS. 4A-4D, following which control gates 43 are used as a mask inthe etching of dielectric film 49 and the poly-1 layer 48 to formfloating gates 41, as illustrated in FIG. 7A. With the control gates asa mask, the edges 41 c, 41 d of the floating gates are aligned with theedges 43 a, 43 b of the control gates. Thereafter, as illustrated inFIG. 7B, oxide spacers 62 are formed around the select gates and thecontrol gates by depositing an oxide film and then etching it awayanisotropically in the flat areas. As in the previous embodiment, thespacers surround the floating gates as well as the control gates. Sourceand drain regions 56-58 are formed by ion-implantation, and the sourcejunctions 58 is made deeper to withstand the high voltages that areapplied to the source nodes during erase operations.

FIGS. 8A and 8B show cross-sections of the embodiments of the memorycell arrays of FIGS. 3 and 6 utilizing shallow trench and LOCOSisolation for aligning the edges 41 a, 41 b of the floating gates withthe edges 42 a, 42 b of the active areas. Those techniques are describedin detail in Ser. No. 09/255,360, the disclosure of which isincorporated herein by reference.

In the embodiment illustrated in FIG. 8A, shallow trenches 63 are formedin the silicon substrate 46, and an isolation oxide 64 is deposited inthe trenches and planarized. When the poly-1 layer 48 is deposited, itcovers the isolation oxide as well as the thermal oxide 47, and when itis etched to form the floating gates, it remains on the side walls ofthe isolation oxide as well as on the thermal oxide. Thus, the floatinggates have side walls 41 e and bottom walls 41 f, with the height of theside walls being on the order of 80 to 160 percent of the width of thebottom walls. The control gates 43 extend into the regions bounded bythe side walls, and the areas of the side walls add significantly to thecapacitance between the gates.

Since the trenches in which the isolation oxide is formed define theedges 42 a, 42 b of the active areas, the edges 41 a, 41 b of thefloating gates are automatically aligned with those edges when thefloating gates are formed on the sides of the isolation oxide. Again inthis embodiment, the floating gates have side walls 41 e and bottomwalls 41 f, with the height of the side walls being on the order of 80to 160 percent of the width of the bottom walls. By making the poly-1layer thin and having it extend along the side walls as well as thebottom walls of the control gates, the capacitance between the controlgates and the floating gates is made high. By increasing the height 66of the isolation oxide above the surface of the poly-1 material, thecapacitance can be further increased. This results in a large couplingratio between the control gates and the floating gates.

The embodiment of FIG. 8B is similar to the embodiment of FIG. 8A exceptthat it uses LOCOS isolation instead of shallow trenching. In thisembodiment, the isolation oxide 67 is thermally grown to define theedges 42 a, 42 b of the active areas, and the poly-1 layer which formsthe floating gates is deposited over that oxide. Since the floatinggates extend along the side walls of the isolation oxide, the edges 41a, 41 b of the floating gates are automatically aligned with the edgesof the active areas. With the thin poly-1 layer extending along both theside walls and the bottom walls of the control gates, the capacitancebetween the control gates and the floating gates is once again high, andcan be made even higher by increasing the height 69 of the isolationoxide. This again results in a large coupling ratio between the controlgates and the floating gates.

A circuit diagram for the memory cell arrays of FIGS. 3 and 6 is shownin FIG. 9. All of the memory cells in a given column have their drainsconnected to bit lines BL_(n−1), BL_(n), BL_(n+1), etc., which aretypically metal lines 71-73 that cross over the active areas, and areisolated from each other by a dielectric film (not shown). All of thecells in a given row are connected to a source line 74, which istypically an N+ or P+ diffusion layer in the silicon substrate 46. In agiven row, all of the control gates 43 are connected together by theportion of the poly-2 layer 51 a of which they are formed, and all ofthe select gates 44 are connected to a word line comprising the portionof the poly-2 layer 51 b of which they are formed. The control gates andthe select gates cross over the active areas and the isolation oxides.

Operation of the memory cells fabricated in accordance with theprocesses of FIGS. 4A-4F, 5A-5C and 7A-7B is as follows, with biasvoltages applied to the four node terminals as set forth in Table 1.

TABLE 1 Mode Control Gate Select Gate Drain Source Erase (1) 0 voltsFloating Floating 12 to 15 volts Erase (2) −5 to −10 volts FloatingFloating 5 to 10 volts Erase (3) −5 to −10 volts 7 to 12 volts 5 to 10volts Floating Program 8 to 12 volts 6 to 8 volts 5 volts 0 volts (1)Program 12 to 15 volts 0 volts Floating 0 volts (2) Program 12 to 15volts 2 to 5 volts 0 volts Floating (3) Read 3 to 5 volts 1.5 to 3 volts1.5 to 3 volts 0 volts

In the erase mode, electrons are forced to travel from the floatinggates 41 to the overlapped source regions 58 a or the overlapped drainregions 57 a by Fowler-Nordheim tunneling. During erase operations, arelatively high electric field (greater than 10 mV/cm) is establishedacross tunnel oxide 47. Erase paths between the floating gates 41 andthe overlapped source nodes 58 a are established either by applying 0volts to the control gates and about 12 to 15 volts to the source nodes,or by applying a negative voltage of about −5 to −10 volts to thecontrol gates and a positive voltage of about 5 to 10 volts to thesource nodes. Those are the two modes which are designated Erase (1) andErase (2) in Table 1. In both cases, the select gate and the drain nodeare kept floating.

Alternatively, erase paths can be established between the floating gates41 and the overlapped drain nodes 57 a by applying a negative voltage ofabout −5 to −10 volts to the control gates, a positive voltage of about5 to 10 volts to the drain nodes, a positive voltage of about 7 to 12volts to the select gates, and keeping the source nodes floating. Thisis the Erase (3) mode shown in Table 1.

In all of these embodiments, the coupling ratio from the control gate tothe floating gate in the erase mode is typically on the order of 85percent. Accordingly, most of the voltage difference between the sourceor drain and control gates is applied across the tunnel oxide,initiating Fowler-Nordheim tunneling and forcing electrons to migratefrom the floating gates to the overlapped source or drain regions. Afteran erase operation, the floating gates are positively charged, thethreshold voltage of the cell becomes lower, and the cell is in aconducting, or logic “1”, state.

In the program mode, electrons are injected into the floating gates, andthe floating gates become negatively charged. This can be done either byhot carrier injection or by Fowler-Nordheim tunneling. In hot carrierinjection, shown as the Program (1) mode in Table 1, the control gatesare biased about 8 to 12 volts, the select gates are biased at about 6to 8 volts, the drains are biased at about 5 volts, and the sources arebiased at 0 volts. When electrons flow from the sources 58 to the drains57, they are accelerated by the high electric field in the channelregions 42, and some of them become heated near the drain junctions.Some of the hot electrons exceed the oxide barrier height of about 3.1eV and are injected into the floating gates.

Fowler-Nordheim tunneling can be utilized for programming by biasing thenodes in either of the two ways indicated as the Program (2) and Program(3) modes in Table 1. In the Program (2) mode, programming paths areestablished between the floating gates 41 and the overlapped sourcenodes 58 a by applying about 12 to 15 volts to the control gates and 0volts to the source nodes and the select gates at 0 volts, with thedrain nodes floating. In the Program (3) mode, programming paths areestablished between the floating gates and the overlapped drain nodes 57a by applying about 12 to 15 volts to the control gates, 0 volts to thedrain nodes, and about 2 to 5 volts to the select gates, with the sourcenodes floating. Following a programming operation, the floating gatesare negatively charged, the threshold voltage of the cell becomeshigher, and the cell is in a non-conducting, or logic “0”, state.

In the read mode, the control gates are biased to about 3 to 5 volts,the select gates are biased to about 1.5 to 3 volts, the sources arebiased to 0 volts, and the drains are biased to about 1.5 to 3 volts.When a memory cell is in an erase state, the read shows a conductingstate, and the sense amplifier reads a logic “1”. When the cell is inthe programming state, the read shows a non-conducting state, and thesense amplifier reads a logic “0”.

When memory cells are constructed in P-wells, a programming operationusing Fowler-Nordheim tunneling can be performed by applying 0 volts tothe P-well nodes and about 12 to 18 volts to the control gates, with thesource and drain nodes floating. In this mode, electrons migrate fromthe channel regions 42 to the floating gates 41, and the floating gatesbecome negatively charged.

It is apparent from the foregoing that a new and improved memory celland fabrication process have been provided. While only certain presentlypreferred embodiments have been described in detail, as will be apparentto those familiar with the art, certain changes and modifications can bemade without departing from the scope of the invention as defined by thefollowing claims.

What is claimed is:
 1. In a memory cell: a substrate having an activearea with source and drain regions on opposite sides of the active area,a floating gate which overlies the active area with portions of thefloating gate extending laterally beyond the active area and overlyingportions of the source and drain regions, a control gate positioneddirectly above the floating gate, and a select gate positioned entirelyto one side of the floating gate and the control gate.
 2. The memorycell of claim 1 wherein the floating gate has a bottom wall and sidewalls which face corresponding walls of the control gate in capacitivecoupling relationship, with the height of the side walls being on theorder of 80 to 160 percent of the width of the bottom wall.
 3. Thememory cell of claim 2 wherein the bottom wall and the side walls of thefloating gate have a thickness no greater than about 1000 Å.
 4. Thememory cell of claim 1 wherein the floating gate is wider than thecontrol gate.
 5. The memory cell of claim 1 wherein an erase window isformed between the source region and the portion of the floating gatewhich overlies it.
 6. The memory cell of claim 1 wherein the sourceregion has a greater junction depth than the drain region.
 7. The memorycell of claim 1 wherein the control gate, the select gate and the sourceand drain regions are biased to establish a Fowler-Nordheim tunnelingpath from the floating gate to the source region during an eraseoperation.
 8. The memory cell of claim 1 wherein the control gate, theselect gate and the source and drain regions are biased to establish aFowler-Nordheim tunneling path from the floating gate to the drainregion during an erase operation.
 9. The memory cell of claim 1 whereinthe control gate, the select gate and the source and drain regions arebiased to establish a Fowler-Nordheim tunneling path from the sourceregion to the floating gate during a programming operation.
 10. Thememory cell of claim 1 wherein the control gate, the select gate and thesource and drain regions are biased to establish a Fowler-Nordheimtunneling path from the drain region to the floating gate during aprogramming operation.
 11. The memory cell of claim 1 wherein thecontrol gate, the select gate and the source and drain regions arebiased to establish hot carrier injection from the active region to thefloating gate during a programming operation.
 12. In a process offabricating a memory cell, the steps of: forming an active area in asubstrate, forming a floating gate which overlies the active area withportions of the floating gate extending laterally beyond the activearea, forming a control gate directly above the floating gate, forming aselect gate entirely to one side of the floating gate and the controlgate, and forming source and drain regions on opposite sides of theactive area with the laterally extending portions of the floating gateoverlying portions of the source and drain regions.
 13. The process ofclaim 12 wherein the floating gate is formed with a bottom wall and sidewalls which face corresponding walls of the control gate in capacitivecoupling relationship, with the height of the side walls being on theorder of 80 to 160 percent of the width of the bottom wall.
 14. Theprocess of claim 13 wherein the bottom wall and the side walls of thefloating gate are formed with a thickness no greater than about 1000 Å.15. The process of claim 12 wherein the floating gate is formed widerthan the control gate.
 16. The process of claim 12 wherein an erasewindow is formed between the source region and the portion of thefloating gate which overlies it.
 17. The process of claim 12 wherein thesource region is formed with a greater junction depth than the drainregion.
 18. The process of claim 12, including the steps of biasing thecontrol gate, the select gate and the source and drain regions toestablish a Fowler-Nordheim tunneling path from the floating gate to thesource region during an erase operation.
 19. The process of claim 12,including the steps of biasing the control gate, the select gate and thesource and drain regions to establish a Fowler-Nordheim tunneling pathfrom the floating gate to the drain region during an erase operation.20. The process of claim 12, including the steps of biasing the controlgate, the select gate and the source and drain regions to establish aFowler-Nordheim tunneling path from the source region to the floatinggate during a programming operation.
 21. The process of claim 12,including the steps of biasing the control gate, the select gate and thesource and drain regions to establish a Fowler-Nordheim tunneling pathfrom the drain region to the floating gate during a programmingoperation.
 22. The process of claim 12, including the steps of biasingthe control gate, the select gate and the source and drain regions toestablish hot carrier injection from the active region to the floatinggate during a programming operation.
 23. In a memory cell: a substratehaving an active area with source and drain regions of differentjunction depth on opposite sides of the active area, a floating gatewhich overlies the active area with portions of the floating gateextending laterally beyond the active area and overlying portions of thesource and drain regions, a control gate positioned directly above thefloating gate, a select gate positioned to one side of the floating gateand the control gate, and additional source and drain regions formed inthe substrate toward opposite sides of the select gate.
 24. The memorycell of claim 23 wherein the source region has a greater junction depththan the drain region.
 25. In a process of fabricating a memory cell,the steps of: forming an active area in a substrate, forming a floatinggate which overlies the active area with portions of the floating gateextending laterally beyond the active area, forming a control gatedirectly above the floating gate, forming a select gate entirely to oneside of the floating gate and the control gate, forming source and drainregions of different junction depth on opposite sides of the active areawith the laterally extending portions of the floating gate overlyingportions of the source and drain regions, and forming additional sourceand drain regions in the substrate toward opposite sides of the selectgate.
 26. The process of claim 25 wherein the source region is formed toa greater junction depth than the drain region.
 27. In a memory cell: asubstrate; a stack transistor formed on the substrate, comprising anactive area in the substrate with source and drain regions on oppositesides of the active area, a floating gate which overlies the active areawith portions of the floating gate extending laterally beyond the activearea and overlying portions of the source and drain regions, and acontrol gate positioned directly above the floating gate; and a selecttransistor formed on the substrate to one side of the stack transistor,comprising a select gate positioned entirely to the side of the stacktransistor, and additional source and drain regions formed in thesubstrate toward opposite sides of the select gate.
 28. The memory cellof claim 27 wherein the source region in the stack transistor has agreater junction depth than the drain region.